1. Field of the Invention
The present invention relates to a method and a system for locating and tracking the location of a knife-edge, such as the edge of a shutter blade in an optical system.
2. Description of the Related Art
Knife-edges are used in various apparatus, such as, an optical apparatus. In various such applications, the exact location and movement of the knife-edge needs to be determined accurately within the image plane. One example is an optical shutter, where the knife-edge may define the field of view of the optical system. In order to automate a shutter system, the location and movement of the shutter needs to be located and monitored automatically.
An example of an optical apparatus that uses a shutter is depicted in FIG. 1. In FIG. 1, and object to be inspected, 100, e.g., a semiconductor integrated circuit, is illuminated through and imaged by collection optics 120. That is, light from light source 130 is passed through field lens 140, shutter 150 (both the field lens and the shutter are at an intermediate image plane), partially-reflecting mirror 160, and into collection optics 120, so as to illuminate particular area of the object 100. Light reflected from the object 100 is collected by optics 120, passed through partially-reflecting mirror 160 and is imaged by imager 145. The images are sent to computer 180. The entire arrangement is positioned on mechanical stage 175, which is controlled by computer 180.
In order to select a particular area of the object 100 to be imaged, shutter 150 is adjusted so that only that particular area is illuminated. More specifically, shutter 150 is mechanized and a user may control its position via computer 180. That is, an image is taken and, after inspecting the image, the user may decide to increase or decrease the field of view. The user then enters a corresponding command to the computer, and the computer causes the shutter to move a particular amount according to the entries made by the user.
As can be appreciated having the user adjust the field of view in such a step-wise, trial and error method is time consuming and inaccurate. Moreover, once the particular field of view has been achieved and an image taken, it may still be unclear what is the exact size of the filed of view. Additionally, if a second picture of another location is needed to be at the same field of view, the entire sequence needs to be repeated and, even then it is not clear that the exact same field of view has been achieved. Finally, as can be understood, having the user verify the field of view prevents the system from being fully automated.
There are mechanical systems for monitoring the position of objects, such as linear encoders. However, such encoders only measure distance relative to the mechanical assembly in the intermediate image plane, not in the plane of the imaging detector itself where the image is detected.
What is needed is means to track the blade position using the image. With such a system a user could, for example, draw a box on the system's display screen and have the shutters move so that the edges of the shutters coincide with the box drawn by the user. The user could also adjust the edges of the box spatially and have the shutters track the positions of the drawn box edges.
What is also needed is a way to detect the shutter blade positions in the image plane.